The long-term goals of the research in the Ochman laboratory are to investigate the origins and ancestry of virulence characters in bacterial pathogens. Enteric bacteria display considerable diversity in metabolic properties, host range and patterns of virulence, and the extensive genetic and phenotypic variation suggests that large-scale changes have occurred within the genomes of these species since they diverged. The proposed studies on bacterial virulence stems from previous work on the organization and evolution of bacterial genomes but represents a major change in both the focus and the experimental procedures applied by this laboratory. This research addresses two questions fundamental to the study of microbial pathogenesis: (l) How do virulence attributes arise in bacterial genomes, and (2) What are the genetic factors that differentiate the host range, pathogenic potential and invasion mechanisms of closely-related organisms? An RCDA will allow the candidate to move into a new and emerging field of research, and acquire skills necessary to investigate molecular and genetic aspects of bacterial pathogenesis. And in addition to directly enhancing the research of the applicant, the award will foster the development of an interdisciplinary program at the University of Rochester concerning the interactions between parasites and hosts at several levels of biological organization. Many enteric pathogens, as well as some plant pathogens, utilize a novel protein transport system to secrete the proteins promoting their entry into host cells, and based on the structure, organization, and distribution of the genes encoding this secretory apparatus, it is apparent that, in many species, these sequences were acquired through horizontal transfer. The discovery that related gene clusters direct host cell entry by divergent pathogens provides a unique opportunity to examine both the patterns of evolution in genes common to invasive microorganisms and the effects of cross-species complementation to address issues concerning the role of specific sequences in host cell entry. The objectives of the research are to establish: (1) the origin and organization of virulence-associated loci in divergent species; (2) the ancestry of the Sequences responsible for cell entry by Salmonella and Shigella; (3) the relationship between the diversification in antigen export pathways and the invasion mechanisms of related organisms, and (4) the role of invasion genes in non-pathogenic organisms. Functional analysis of these sequences, accomplished by introducing conserved and divergent components of this export pathway into different genetic backgrounds, provides information about the manner in which these transport proteins have diversified to interact with particular substrates or hosts.